[Perf] Async GPU P2P access cache precomputation to reduce startup time#39249
[Perf] Async GPU P2P access cache precomputation to reduce startup time#39249mrigasiyer wants to merge 3 commits into
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When VLLM_SKIP_P2P_CHECK=0, the P2P access check blocks startup for ~60s on multi-GPU CUDA configs. This overlaps that work with model weight loading. - Extract subprocess work into _precompute_p2p_access_cache() (thread-safe) - Defer CustomAllreduce P2P check to finalize_p2p() (collective, called before first allreduce) - Add finalize_p2p_check() to base communicator and CudaCommunicator - Launch background thread in Worker.init_device() on local_rank==0 - Join thread and finalize collectively in determine_available_memory() Partially addresses vllm-project#19824
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This pull request optimizes the GPU P2P access check by pre-computing the cache in a background thread during model loading, which helps reduce startup latency. It introduces a deferred initialization mechanism for custom all-reduce, allowing the P2P check to overlap with other initialization tasks. Feedback was provided to ensure the atomicity of the cache file write operation to prevent potential file corruption if the process is interrupted.
Write to a temp file then os.replace() to avoid leaving a corrupted JSON if the process is interrupted mid-write.
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Purpose
Fix startup latency regression when
VLLM_SKIP_P2P_CHECK=0: the GPU P2P access check blocks startup for ~60s on multi-GPU CUDA configs because it spawns subprocesses to verify IPC memory access across all GPU pairs before custom allreduce can initialize.This PR overlaps that subprocess work with model weight loading by running it in a background thread, making it effectively free on any model that takes longer than ~60s to load (most production models at TP≥2).
Partially addresses #19824.
This is not a duplicate of any open PR — checked against open PRs searching for "19824 in:body", "p2p async startup", and "gpu_p2p_access_check async". The related PR #39040 addresses a different aspect (enabling custom allreduce on PCIe topologies).
Test Plan
Ran linter on all changed files:
Test Result
Essential Elements of an Effective PR Description Checklist
supported_models.mdandexamplesfor a new model.